Linebreak in Simplified Chinese

I have two questions that are related to Simplified Chinese.

1) I find the linebreaks have a problem. Adding
\XeTeXlinebreaklocale "en" solves most of the probelm, but
 some lines still have
zigzag endings. Change "en" to "zh-cn" does not help.
 What else can I do here?

2) How can I let section heading into large bold faces? 
 Below is my sample latex file.

Thanks.
JD


%!TEX TS-program = xelatex
%!TEX encoding = UTF-8 Unicode


\documentclass[11pt]{article}
\usepackage[letterpaper,margin=1.0in]{geometry} 
\usepackage{color}
\addtolength{\parskip}{5pt}

\usepackage{fontspec}
\setromanfont[Scale=1.0]{Kai Regular} 
%\setboldfont[scale=1.2]{Hei Regular} 
\setmonofont[Scale=0.8]{Kai Regular} 
\XeTeXlinebreaklocale "zh-cn"

\begin{document}

\section{Introduction}

我们怎么才能做第一件事都荣耀神呢？只要把每件事都看成是为主而做的，
\end{document}